1. Field of the Invention
The present invention relates to an apparatus which inspects for defects, and in particular, to an apparatus that inspects a mask pattern for defects.
2. Description of the Related Art
With an increase in the degree of integration of semiconductor devices such as LSIs, efforts are being made to reduce the sizes of mask patterns formed on masks such as reticles. This requires high performance to be exhibited by defect inspecting apparatuses that inspect a pattern created on a mask for defects. However, it is not always easy to implement a high-performance defect inspecting apparatus.
The problems described below may occur in the inspection of a mask pattern for defects, for example, in the inspection of a mask pattern in which the phase of an optical wave shifts on a mask in accordance with a pattern. Masks on which a phase shift occurs include those in which a shift material that shifts phase is formed in a shifter section that shifts the phase of an optical wave, to provide the shifter section with a phase difference of λ/2, and those in which a glass substrate is trenched in the shifter section to provide the shifter section with a phase difference of λ/2. In particular, in the trench type phase shift mask, a trench region has almost the same transmittance as that of an un-trenched region. Accordingly, it is difficult to increase the contrast of the boundary between the trench region and the un-trenched region. Therefore, it is necessarily difficult to increase the detection sensitivity of the defect inspecting apparatus.
In connection with this problem, Jpn. Pat, Appln. KOKAI Publication No. 10-177246 proposes a defect inspecting apparatus utilizing a differential interference optical system. The differential interference optical system is implemented utilizing a birefringence prism that separates a bundle of light rays into smaller bundles separated from each other by a very small angle. Specifically, the birefringence prism separates a bundle of transmitted light rays or reflected light rays from a mask into a bundle of ordinary rays and a bundle extraordinary rays separated from each other by a very small angle. The two bundles of light rays resulting from the separation interfere with each other at an image formed surface. The intensities of the bundles of light rays vary depending on a phase difference. Since the phase difference between the trench region and un-trenched region of the phase shift mask is λ/2, the intensity varies significantly at the boundary between the trench region and the un-trenched region. Consequently, the boundary of the shifter section can be extracted very sensitively.
With the above method, if a separating direction, that is, a differential direction, at the birefringence prism is perpendicular to the boundary line between the trench region and the un-trenched region, the differential interference effect significantly varies the intensity at the boundary line. This enables the boundary to be detected very sensitively. However, if the bundle-of-light-ray separating direction is parallel to the boundary line, the intensity does not vary at the boundary line. This prevents the boundary line from being detected. Thus, it is difficult to very sensitively detect all the boundaries between the trench regions and the un-trenched regions using the above method. Disadvantageously, it is difficult to reliably detect defects in the shifter boundary for all the directions.
As described above, a problem with the conventional mask pattern defect inspecting apparatus is that it is difficult to reliably detect defects in a pattern for all the directions.